Optical diffusers are constituted by an assembly of 2 by 2 couplers, i.e. couplers having two inlets and two outlets, with such couplers being of any conventional type known to the person skilled in the art.
The principle of assembling couplers 2 by 2 in multiple stage networks in order to realize high capacity diffusers is, itself, known: the article by M. E. Marhic entitled "Hierarchic and combinatorial star couplers" published in Optics Letters, Vol. 9, No. 8, August 1984, pages 368 to 370 describes one possible assembly of elementary (2 by 2) couplers in a welded mesh, however a diffuser constituted in this way suffers from high loss due to the accumulation of the loss due to the successive mesh connections. One way of reducing this loss between the inlet and the outlet of the diffuser consists in reducing the number of interconnections by directly combining 2.sup.n lengths of fiber in order to constitute the couplers of a diffuser. However, the author describes neither a practical organization nor a method of making such diffusers. The article by D. B. Mortimore entitled "Low-loss 8.times.8 single-mode star coupler" published in Electronics Letters, May 1985, Vol. 21, No. 11, pages 502 to 504 describes an 8 by 8 star type coupler constituted by three rows of four couplers.
N by N diffusers have a plane structure and are realized by interconnecting two 1/2N by 1/2N diffusers by means of couplers; for example an 8 by 8 diffuser is realized by interconnecting two 4 by 4 diffusers via four couplers.
Prior art diffusers made using lengths of fiber and occupying a plane structure suffer from several drawbacks:
the fiber outlets are not ordered, i.e. they do not succeed one another in the same order at the outlet as at the inlet;
fiber lengths differ from one length to another and consequently signal propagation times and losses are not identical; and
the various paths are different and complex such that for a large number of fibers it is not possible to fabricate a diffuser automatically in simple manner.
The object of the invention is to provide an N by N diffuser based on N=2.sup.n light ducts and in which the light ducts all have the same length between the inlet and the outlet of the diffuser.
Another object of the invention is to provide an N by N diffuser in which the light ducts follow identical paths.
Another object of the invention is to provide an N by N diffuser in which the order in which the light ducts succeed one another at the outlet of the diffuser is the same as the order in which they succeed one another at its inlet.
Another object of the invention is to provide and N by N diffuser capable of being manufactured automatically by a machine derived from a conventional cable-making machine.